1. Field of the Invention
The present invention relates to an image processing apparatus which forms an image on a conveyed sheet or reads an image of a conveyed original.
2. Description of Related Art
A conventional image forming apparatus which forms an image on a sheet such as a copying machine and a printer and a facsimile machine and a conventional image reading apparatus which reads an image of an original such as a scanner include a sheet conveying apparatus which conveys a sheet or an original.
In the case of the sheet conveying apparatus of the image processing apparatus such as the image forming apparatus and the image reading apparatus and it is necessary to straightly send a sheet or an original to the image forming portion of the image forming apparatus or the image reading portion of the image reading apparatus. If the sheet conveying apparatus obliquely sends a sheet to the image forming portion, the image forming apparatus obliquely forms an image on the sheet. If the sheet conveying apparatus obliquely sends an original to the image reading portion, the image reading portion obliquely reads the original. Thus, the sheet conveying apparatus includes a registration apparatus which registers the skew feeding before an inclined (skew fed) sheet or original is sent to the image forming portion or the image reading portion.
As systems of the registration apparatus, there are a loop registration system and a shutter registration system. According to the loop registration system, a tip end of a sheet is butted against a nip of a pair of stopped rollers to form the sheet with a warp and the skew feeding of the sheet is registered utilizing rigidity of the sheet. According to the shutter registration system, a tip end of a sheet is butted against a shutter member provided in a sheet conveying passage and then, the shutter member is retracted from the sheet conveying passage.
In recent years, however, with the digitization of the image forming apparatus and the image reading apparatus, there is a tendency that a distance between sheets or originals (“sheet-gap”, hereinafter) is reduced. The reason why the sheet-gap is reduced is that more sheets can be processed within a short time and substantial image forming speed can be enhanced without increasing the processing speed of image formation, and that more originals can be read within a short time without increasing the reading speed of originals.
Therefore, time required for registration is one of the factors which determine the sheet-gap. As one of methods for shortening the time required for registration and there is an active registration method in which skew feeding of sheets is corrected while conveying sheets. According to this method, a tip end of a conveyed sheet is detected by two sensors disposed in a sheet conveying passage in a direction intersecting with a sheet conveying direction, and when the sheet is skew fed, a control portion calculates the inclination of the tip end of the sheet based on a difference in detection time. Then, a sheet conveying speed of a plurality of skew feeding correction rollers which are disposed in a direction intersecting with the sheet conveying direction and which rotates independently are controlled on the calculated skew feeding amount and the sheet is turned to correct the skew feeding (see Japanese patent Application Laid-open No.H8-108955).
According to this active registration method, since the skew feeding can be corrected while conveying a sheet, reduce the sheet-gap as compared with the loop registration method and the shutter registration method in which a sheet is once stopped.
A sheet conveying apparatus in which the active registration method is employed will be described based on FIG. 24. A sheet conveying apparatus 900 includes a tip end detection sensor 904 which detects a tip end of a sheet, a pair of conveying rollers 905 and 906, skew feeding detection sensors 913 and 914 which detect skew feeding of a sheet, and a pair of registration rollers 907 and 908 which correct the skew feeding of a sheet. The pair of conveying rollers 905 and 906 includes a drive rollers 905a and 906a which are rotated by a motor 910 and which have half-moon shaped cross sections, and follower rollers 905b and 906b which follow the drive rollers 905a and 906a. 
If the tip end detection sensor 904 detects the tip end of a sheet, the drive rollers 905a and 906a rotate by predetermined number of times and sends a sheet into the pair of registration rollers 907 and then, the drive rollers 905a and 906a stop at a position where nip between the follower rollers 905b and 906b is released. At that time, the drive rollers 905a and 906a rotate in a state where the directions of the half-moon shapes match with each other (the same phases).
Each of the pair of registration rollers 907 and 908 comprises a pair of rollers which independently rotate in a direction intersecting with the sheet conveying direction. The pair of registration rollers 907 and 908 turn a sheet such that sheet conveying speeds of the pair of rollers disposed in the direction intersecting with the sheet conveying direction are different from each other, and correct the skew feeding of the sheet with the skew feeding amount detected and calculated by the skew feeding detection sensors 913 and 914. The nip of the pair of conveying rollers 905 and 906 of the sheet which is turned by the pair of registration rollers 907 and 908 and whose skew feeding is corrected is released. Therefore, no load is added from the pair of conveying rollers 905 and 906, the skew feeding is corrected smoothly.
With this, the sheet conveying apparatus 900 can continuously convey a sheet while maintaining the correcting precision of the skew feeding at high level without restraining a rear end side of the sheet by the pair of conveying rollers 905 and 906. The structure of the sheet conveying apparatus 900 can also be applied to an image reading apparatus which convey an original.
As described above, correction of the skew feeding of a sheet is extremely important to enhance the forming precision of an image or the reading precision of an image of the image forming apparatus or the image reading apparatus. In the case of the active registration method using the skew feeding correcting rollers which are coaxially disposed and independently rotate, the skew feeding correcting precision is deteriorated depending upon a machining error of the skew feeding correcting rollers in some cases.
The reason is as follows. That is, in the case of the two pairs of skew feeding correcting rollers having machining errors, even if the rollers rotate at isogonal speed, since a circumferential speed of the roller is irregularly varied in the nip portion, the skew feeding correcting rollers irregularly convey a sheet in a meandering manner. Even if the skew feeding amount of a nipped sheet is 0, if the skew feeding correcting rollers pass the sheet to a downstream pair of conveying rollers in a state where the sheet is conveyed in the meandering manner, there is a fear that the skew feeding correcting rollers pass the sheet in the skew feeding state. As a result the skew feeding correcting precision of a sheet of the sheet conveying apparatus 900 is deteriorated.
To solve this problem, it is conceived that all of the skew feeding correcting rollers always maintain positions thereof on their outer peripheries where the rollers start coming into contact with a sheet at the same level (phases of the pair of registration rollers are aligned with each other), thereby preventing the skew feeding correcting precision from being deteriorated. That is, if the meandering states of sheets conveyed by the skew feeding correcting rollers are brought into exact agreement every time, the variation of the skew feeding amount is reduced. If the fact that a constant amount of skew feeding is generated in a sheet is previously found, reduce the amount of skew feeding by carrying out the skew feeding correcting control in which the control amount is corrected correspondingly.
The skew feeding correcting precision of a sheet is pertinent to a conveying load of a sheet and a conveying force of the skew feeding correcting rollers. It is necessary to always satisfy the relation of a conveying load of a sheet<a conveying force of the skew feeding correcting rollers.
If the relation of a conveying load of a sheet>a conveying force of the skew feeding correcting rollers, it is impossible to correct the skew feeding. Even if the conveying load of a sheet and the skew feeding correcting rollers are close to each other, a slip is generated between the sheet and the skew feeding correcting rollers, and the skew feeding can not be corrected sufficiently.
The conveying load of a sheet is mainly generated when the sheet slides on a conveying path. Thus, the conveying load of a sheet can be reduced to some degree if the sheet can easily slide on a conveying path where skew feeding of sheet is corrected.
In the case of the sheet conveying apparatus of the active registration system, a conveying load of a sheet is reduced. Thus, there is proposed a sheet conveying apparatus in which an upstream conveying roller of the skew feeding correcting roller is configured such that it can move in a direction intersecting with the sheet conveying direction, it follows the turning motion when the skew feeding of a sheet is corrected and a load at the time of turning is reduced (see Japanese patent Application Laid-open No. H10-175752).
In the conventional sheet conveying apparatus, an upstream side of the skew feeding correcting roller is formed into a shape or structure in which a conveying load is small. However, since a conveying load applied to the skew feeding correcting roller can not be 0, a slip may be produced between the skew feeding correcting roller and a sheet in some cases. Further, when a sheet is turned by a speed difference between the skew feeding correcting rollers, since the sheet is turned in the roller nip, a slip is generated between the skew feeding correcting roller and the sheet in a rotation direction of the sheet. Since an inertial force of a sheet is applied to the slip phenomenon generated between the skew feeding correcting roller and the sheet, the slip phenomenon appears most seriously at an instant when a turning acceleration is applied to the sheet.
When control is performed (phase control) such that all of the skew feeding correcting rollers always maintain positions thereof on their outer peripheries where the rollers start coming into contact with a sheet at the same level, slip phenomena are concentrated on the same position on the outer periphery of the skew feeding correcting roller. Therefore, there is a problem that the same portions on the outer peripheries of the skew feeding correcting rollers are worn, a conveying force of a sheet is largely deteriorated at the worn portion, and a conveying amount of a sheet is reduced, and the skew feeding correcting precision is deteriorated. This problem is not limited to the skew feeding correcting roller, and the same problem is also generated when the conveying roller which conveys sheets in the sheet conveying direction while accelerating or decelerating the sheets is controlled in phase.
Hence, in a conventional sheet conveying apparatus having the conventional skew feeding correcting rollers or conveying rollers whose phases are controlled, the rollers are exchanged periodically in accordance with reduction in conveying force of the skew feeding correcting rollers or conveying rollers to maintain the sheet conveying precision such as the skew feeding correcting precision. However, if the rollers are exchanged periodically, cost is increased.
The image forming apparatus having the sheet conveying apparatus of the active registration system is a high speed machine in which the number of images to be formed per unit time is high in many cases, and the image forming apparatus is used commercially in many cases. Thus, if the stop time required for maintenance in which the skew feeding correcting rollers or conveying rollers are exchanged is long, this deteriorates the image forming efficiency.
In recent years, sheets to be used are diversified from thin sheet to thick sheet from small size of about postcard size to large size of about 330 mm×488 mm, and image forming apparatuses which cope with the diversification are increased. Thus, in the case of the sheet conveying apparatus, the skew feeding correcting roller is prone to be worn locally when a thick sheet or large size sheet is conveyed, the conveying force is reduced, the number of exchanging operations of the skew feeding correcting rollers or conveying rollers is increased, and a serviceability ratio is deteriorated.
As described above, it is required that lifetime of a rotation body such as the skew feeding correcting roller and the conveying roller whose phase is to be control is increased.
The present invention prevents a sheet conveying force of a rotation body such as a skew feeding correcting roller and a conveying roller from being reduced locally, reduces the deterioration in the skew feeding correcting precision, and increases the lifetime of the rotation body.